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Pitch-Chain Gearing.                         545

two sets of friction surfaces; the teeth on the pin and the pin on
the inner links. B decreases the friction of the former surfaces by
the introduction of rollers, and at c the latter surfaces are enlarged
by riveting a ferrule to the inner links. The pins should be
shouldered, so that the links may work clear at the sides; and
the teeth are involute curves having the arc of pin centres as base

D is a road roller supplied with pitch chain, and Fig. 569 an
electric car driven by chain from a dynamo. Cycle driving is a
well-known application. (Seef.nix.)

(ii.) Compressed Air is of great advantage as a long-
distance transmitter, and as such has been used for motors in
mines, for tunnelling machines, and for distribution from central
stations in towns.    In mines and workshops, the exhaust serves
^ also for ventilation.   To be effective the compressors should be
f on a somewhat large scale, and the arrangement is shewn in Fig.
561, where steam in cylinders A A is used to compress air in the